Several nucleating agents for thermoplastic polymers are known in the art. These nucleating agents generally function by forming nuclei or providing sites for the formation and/or growth of crystals in the thermoplastic polymer as it solidifies from a molten state. The nuclei or sites provided by the nucleating agent allow the crystals to form within the cooling polymer at a higher temperature and/or at a more rapid rate than the crystals will form in the virgin, non-nucleated thermoplastic polymer. These effects can then permit processing of a nucleated thermoplastic polymer composition at cycle times that are shorter than the virgin, non-nucleated thermoplastic polymer. The nuclei or sites provided by the nucleating agent may also reduce the size of the spherulites formed upon cooling of the polymer, which is believed to improve the optical properties (e.g., reduce the haze levels) exhibited by articles formed from the polymer.
While polymer nucleating agents may function in a similar manner, not all nucleating agents are created equal. For example, a nucleating agent may be effective at increasing the peak polymer recrystallization temperature of a thermoplastic polymer and produce a molded part exhibiting relatively low, isotropic shrinkage, but such a nucleating agent may negatively affect the optical properties (e.g., haze and/or clarity) of the thermoplastic polymer, rendering the nucleating agent ineffective for use in producing articles that must exhibit low haze and/or high clarity. Also, while nucleating agents for polyethylene polymers are known in the art, relatively few of these nucleating agents have been shown to improve the optical properties of the polyethylene polymer to any appreciable degree.
Given the complicated interrelationship of these properties and the fact that many nucleating agents exhibit less-than-optimal behavior for at least one of these properties, a need remains for nucleating agents that are capable of producing thermoplastic polymer compositions exhibiting a more desirable combination of high peak polymer crystallization temperature, improved optical properties, and high stiffness. In particular, a need remains for nucleating agents that are capable of improving the optical properties (e.g., haze and/or clarity) of polyethylene polymers without negatively impacting the polymer crystallization temperature and flexural properties exhibited by such polymers. Applicants believe that the nucleating agents and thermoplastic polymer compositions disclosed in the present application meet such a need.